1. Field of the Invention
The present invention relates to a circuit-integrating light-receiving element integrating a signal processor circuit for processing a photoelectrically converted signal. More particularly, the present invention relates to a divided photodiode structure which is incorporated into the circuit-integrating light-receiving element and has a light-receiving region divided into a plurality of light-detecting sections (hereinafter, referred to as "light-detecting photodiode sections") for reducing the level of radio frequency noise thereof without decreasing the response speed thereof.
2. Description of the Related Art
Such a circuit-integrating light-receiving element provided with a divided photodiode has conventionally been used as a signal detecting element for an optical pickup, for example.
FIGS. 10A and 10B illustrate a conventional circuit-integrating light-receiving element: FIG. 10A is a plan view showing a general configuration of a quadruple divided photodiode of the circuit-integrating light-receiving element, and FIG. 10B is a view showing a cross-sectional structure thereof taken along the line 10b--10b in FIG. 10A. It is noted that various components including multi-layer wires, a protective film and the like to be formed during the respective steps succeeding a metal processing step are omitted from FIGS. 10A and 10B.
In FIGS. 10A and 10B, PD20 is a quadruple divided photodiode incorporated into a conventional circuit-integrating light-receiving element, and an N-type epitaxial layer 14 is formed on a P-type semiconductor substrate 11 thereof. In the boundary region between the substrate 11 and the N-type epitaxial layer 14, a P.sup.+ -type buried diffusion layer 12 is selectively formed. In the surface region of the N-type epitaxial layer 14, a P.sup.+ -type isolating diffusion layer 15 is formed so as to reach the P.sup.+ -type buried diffusion layer 12. Moreover, the N-type epitaxial layer 14 is divided into a plurality of N-type epitaxial regions 14a by the P.sup.+ -type buried diffusion layer 12 and the P.sup.+ -type isolating diffusion layer 15 linked with the diffusion layer 12. Herein, light-detecting photodiode sections PDa, PDb, PDc and PDd for detecting signal light are formed by the respectively divided N-type epitaxial regions 14a and the underlying regions of the substrate 11. And the quadruple divided photodiode PD20 is made up of these light-detecting photodiode sections PDa to PDd. In actuality, an antireflection film such as a silicon oxide film or a silicon nitride film is formed over the surface of the N-type epitaxial layer 14. However, for the sake of simplicity, such a film is not shown in FIG. 10B.
Next, a method for detecting a focus error using the quadruple divided photodiode PD20 will be described.
FIGS. 11A through 11C illustrate the appearances of light beam spots formed on the surface of the quadruple divided photodiode PD20 upon the irradiation of signal light thereto in an astigmatism method as an exemplary method for detecting a focus error using the quadruple divided photodiode PD20 consisting of the light-detecting photodiode sections PDa through PDd.
FIG. 11A illustrates the shape of a light spot formed on the surface of the quadruple divided photodiode when the light beam incoming from an optical pickup is in focus on a disk, FIG. 11B illustrates the shape of a light spot formed on the surface of the quadruple divided photodiode when a distance between the disk and the optical pickup is too small, and FIG. 11C illustrates the shape of a light spot formed on the surface of the quadruple divided photodiode when the distance between the disk and the optical pickup is too large.
In general, a focus error is detected by obtaining the respective sums of the optical signals (i.e., photoelectrically converted outputs) of two pairs of light-detecting photodiode sections located on the diagonals and then examining a difference between the two sum signals thus obtained. Specifically, in this case, the error signal S is calculated as follows:
Error signal S={(optical signal of light-detecting photodiode section PDa)+(optical signal of light-detecting photodiode section PDd)}-{(optical signal of light-detecting photodiode section PDb)+(optical signal of light-detecting photodiode section PDc)} PA0 RF signal={(optical signal of light-detecting photodiode section PDa)+(optical signal of light-detecting photodiode section PDd)}+{(optical signal of light-detecting photodiode section PDb)+(optical signal of light-detecting photodiode section PDc)} PA0 i.sub.n : output current of an input-converted noise current source with respect to the entire signal processor circuit (complex number) PA0 i.sub.na : output current of the input-converted noise current source with respect to the feedback circuit (complex number) PA0 v.sub.na : output voltage of an input-converted noise voltage source with respect to the feedback circuit (complex number) PA0 .DELTA.f: frequency band of a signal to be processed by the signal processor circuit PA0 Rf: resistance of the feedback circuit (feedback resistance) PA0 C.sub.PD : capacitance of the photodiode PA0 k: Boltzmann constant PA0 T: absolute temperature PA0 .omega.=2.pi.f: angular velocity (rad/sec.) PA0 f: frequency of optical signal
For example, when a circular light spot 10a is formed as shown in FIG. 11A, S=0 and it is determined that the light beam is in focus on the disk. On the other hand, when an ellipsoidal light spot 10b inclined counterclockwise with respect to the vertical direction of the divided photodiode is formed as shown in FIG. 11B, S&gt;0 and it is determined that the disk is too close to the optical pickup. Furthermore, when an ellipsoidal light spot 10c inclined clockwise with respect to the vertical direction of the divided photodiode is formed as shown in FIG. 11C, S&lt;0 and it is determined that the disk is too distant from the optical pickup.
When an optical pickup is actually used, a focusing control is performed on a light beam so that S=0. Thus, as shown in FIGS. 11A through 11C, the light beam of signal light is irradiated onto the division section of the divided photodiode.
After the focal distance from the optical pickup to the disk has been adjusted in such a manner, an RF signal including data actually read out from the disk is obtained as the sum of the optical signals (photoelectrically converted signals) from the respective light-detecting photodiode sections in the state where the light beam is irradiated onto the divided photodiode as shown in FIG. 11A. That is to say,
Furthermore, in order to realize high-speed signal processing, irradiated signal light must be converted into an electrical signal at a high speed in a divided photodiode. Thus, as the performance of optical disk drives has been increasingly improved, the response characteristics of a divided photodiode are required to be further improved in an area where signal light is irradiated when the divided photodiode is actually used.
As a result of the need to improve the response characteristics of the division section of a divided photodiode in a state where light is irradiated onto the divided photodiode as shown in FIG. 11A, a structure for improving the response speed in the division section has already been developed (see Japanese Laid-Open Publication No. 8-32100 (Japanese Patent Application No. 6-162412)).
FIG. 12 is a view for illustrating such a structure for improving the response speed in the division section and shows a cross-sectional structure corresponding to the part of a quadruple divided photodiode PD20 shown in FIG. 10B. It is noted that an oxide film or a nitride film as an antireflection film and various components including multi-layer wires, a protective film and the like to be formed during the respective steps succeeding a metal processing step are omitted from FIG. 12.
In FIG. 12, PD30 is a quadruple divided photodiode having such a structure as to improve the response speed in the division section. The divided photodiode PD30 further includes an N.sup.+ -type buried diffusion layer 13 formed in the boundary region between the P-type silicon substrate 11 and the N-type epitaxial regions 14a constituting the respective light-detecting photodiode sections PDa through PDd, in addition to the respective layers and regions of the divided photodiode PD20 shown in FIGS. 10A and 10B. The remaining configuration of the divided photodiode PD30 is the same as that of the divided photodiode PD20.
Next, a method for fabricating this divided photodiode PD30 will be described.
First, as shown in FIG. 13A, a P.sup.+ -type buried diffusion layer 12 is formed in the regions for isolating the divided photodiode from other devices and the region to be the division section among the light-detecting photodiode sections of the divided photodiode in the surface region of the P-type silicon substrate 11. In addition, an N.sup.+ -type buried diffusion layer 13 is formed in a part of the regions in which the light-detecting photodiode sections are to be formed in the surface region of the substrate 11.
Next, as shown in FIG. 13B, an N-type epitaxial layer 14 is grown over the entire surface of the P-type silicon substrate 11. Subsequently, as shown in FIG. 13C, a P.sup.+ -type isolating diffusion layer 15 is formed in the regions corresponding to the P.sup.+ -type buried diffusion layer 12 so as to expand from the surface of the N-type epitaxial layer 14 and to reach the P.sup.+ -type buried diffusion layer 12, and a P.sup.+ -type diffusion layer 16 is further formed in the surface region of the N-type epitaxial layer 14 constituting the divided photodiode.
In this way, the divided photodiode PD30 having a structure such as that shown in FIG. 12 is obtained. Furthermore, a signal processor circuit section (not shown) to be incorporated, together with the divided photodiode, into the circuit-integrating light-receiving element is formed on the P-type silicon substrate 11 by a conventional bipolar IC process.
Next, it will be briefly described how the response characteristics are improved in the division section of the divided photodiode having such a structure.
The divided photodiode PD30 having this structure is characterized by providing the N.sup.+ -type buried diffusion layer 13 for the respective light-detecting photodiode sections and providing the P.sup.+ -type diffusion layer 16 in the surface region of the N-type epitaxial layer 14 constituting the divided photodiode.
First, the reasons why the N.sup.+ -type buried diffusion layer 13 is provided will be described. In the conventional structure for the divided photodiode PD20 shown in FIG. 10B, optical carriers C0 (see FIG. 14A), which have been generated under the division section B to be irradiated with light, reach a P-N junction region after the carriers have made a detour around the division section B. As a result, the distance over which the carriers C0 move by diffusion to the P-N junction region becomes longer than that of optical carriers C1 generated in the regions of the substrate 11 except for the regions under the division section B of the divided photodiode. Consequently, the response speed in the division section B of the divided photodiode PD20 becomes lower than the response speed in the regions other than the division section B and the cutoff frequency in the division section B becomes lower than the cutoff frequency of the regions other than the division section B.
In contrast, in the divided photodiode PD30 having the structure including the N.sup.+ -type diffusion layer 13 as shown in FIG. 13C utilizes a depletion layer expanding from the N.sup.+ -type diffusion layer 13, thereby shortening the distance, over which the optical carriers generated under the division section make a detour to diffuse around the division section, from several tens of .mu.m to several .mu.m. Consequently, the delay of the response speed owing to the optical carriers generated under the division section can be prevented.
From a viewpoint of improving the response speed, it is significant for the divided photodiode PD30 having such a structure to include the N.sup.+ -type diffusion layer 13 in the vicinity of the division section. By modifying the structure of the divided photodiode in such a manner, the response speed can be improved and an RF signal can be processed at a higher speed.
On the other hand, the reasons why the P.sup.+ -type diffusion layer 16 is formed are as follows.
In the case where the light reflectance of the light-receiving surface of the divided photodiode PD30 is high, when signal light is irradiated onto the divided photodiode PD30, smaller amount of light permeates into the divided photodiode PD30. As a result, the amount of current generated by the photoelectric conversion of light into electrical signals becomes smaller. In other words, the photosensitivity of the divided photodiode is decreased.
Thus, in order to prevent the photosensitivity from being decreased in such a manner, it is necessary to decrease the light reflectance on the surface of the divided photodiode. Thus, a silicon oxide film has conventionally been formed as an antireflection film (not shown) on the light-receiving surface of a divided photodiode. However, even if the film thickness of a silicon oxide film is optimized, the silicon oxide film can reduce the reflectance at most to about 15% because of the limitations of the refractive index thereof.
On the other hand, in the case of substituting a silicon nitride film for the silicon oxide film as the antireflection film, the reflectance can be reduced to about 1% by optimizing the film thickness thereof. However, if a silicon nitride film is formed on the surface of the epitaxial layer, then the P-N junction end face between the N-type epitaxial layer 14 and the P.sup.+ -type diffusion layer 15 is in direct contact with the silicon nitride film, so that junction leakage is adversely increased in the P-N junction end face.
Thus, by forming the P.sup.+ -type diffusion layer 16 in the surface region of the epitaxial layer in the divided photodiode as shown in FIG. 14C, it is possible to prevent the P-N junction end face between the N-type epitaxial layer 14 and the P.sup.+ -type diffusion layer 15 from coming into contact with the silicon nitride film, thereby taking measures against the junction leakage in the P-N junction end face.
Hereinafter, the problems of the divided photodiode PD30 having such a structure as that shown in FIG. 14C will be described.
As the operating speed of a divided photodiode used for an optical pickup becomes higher, a signal is processed by a circuit for processing a photoelectrically converted signal obtained by the divided photodiode at a higher frequency. Thus, it is necessary to reduce the level of radio frequency noise in the signal processor circuit of a circuit-integrating light-receiving element.
It has been found that the larger the capacitance of a divided photodiode is, the higher the level of radio frequency noise becomes. The reasons thereof are presumably as follows.
FIG. 15 is a simplified equivalent circuit diagram showing the circuit configuration of a circuit-integrating light-receiving element used for an optical pickup so as to include the components involved with radio frequency noise. In the circuit-integrating light-receiving element, a feedback circuit is provided on a prior stage for a signal processor circuit SC such as an amplifier. Thus, in this equivalent circuit diagram, a capacitance C.sub.PD of the divided photodiode and a resistance Rf of the feedback circuit are connected in parallel to the input of the signal processor circuit SC. In FIG. 15, V denotes a reference voltage of the signal processor circuit SC such as an amplifier.
The noise in such an equivalent circuit can be represented by the following Equation (1). EQU i.sub.n.sup.2 =i.sub.na.sup.2 +v.sub.na.sup.2 /{Rf.sup.2 /(1+.omega.C.sub.PD Rf.sup.2)}+4kT(.DELTA.f/Rf) (1)
In this Equation (1), the variables and the constants are as follows.
In this equation, the first term represents a shot noise, the second term represents radio frequency noise, and the third term represents a thermal noise. Among these terms, the shot noise and the thermal noise do not depend on a frequency.
As can be understood from Equation (1), the second term having a frequency dependence depends upon the load resistance Rf and the capacitance C.sub.PD of the divided photodiode. However, since the load resistance Rf is associated with the amplification factor of the amplifier circuit (i.e., the signal processor circuit) SC, the value of the resistance cannot be varied freely. Thus, in order to reduce the level of the radio frequency noise, it is necessary to reduce the capacitance C.sub.PD of the divided photodiode PD30.
As described above, in the conventional divided photodiode PD30, the response speed can be improved by providing the N.sup.+ -type buried diffusion layer 13 and the reflectance on the light-receiving surface can be reduced without generating a junction leakage, by using a silicon nitride film as an antireflection film, but by forming the P.sup.+ -type diffusion layer 16 in the surface region of the N-type epitaxial layer of the divided photodiode. However, since the junction capacitance between the N.sup.+ -type diffusion layer 13 and the P-type semiconductor substrate 11 and the junction capacitance between the P.sup.+ -type diffusion layer 16 and the N-type epitaxial layer 14 are large, the total junction capacitance of the divided photodiode becomes larger. Thus, the divided photodiode PD30 has a problem in that the level of the radio frequency noise to be generated by the signal processing in the circuit-integrating light-receiving element is adversely high.